Comet begins thaw

The comet 67P/Churyumov-Gerasimenko – target of Europe’s ambitious Rosetta mission – has begun to develop a dust coma.

A sequence of images were taken between 27 March and 4 May as the gap between craft and comet closed from around 5 million km to 2 million km

They show the comet’s dusty veil – the ‘coma’ – extending some 1300 km into space. By comparison, the nucleus is roughly only 4 km across, and cannot yet be ‘resolved’.

The coma has developed as a result of the comet moving progressively closer to the Sun along its 6.5 year orbit.

Even though it is still more than 600 million km from the Sun – more than four times the distance between Earth and Sun – its surface has already started to warm, causing its surface ices to sublimate and gas to escape from its rock-ice nucleus.

Dr Chris Castelli, acting director of programmes at the UK Space Agency (UKSA), said: “Rosetta is a big mission for the UK so we’re very excited to see these first images of comet 67P ‘coming to life’.

“With much of the spacecraft built and designed in the country and UK scientists involved in 10 of the mission’s instruments, we’re looking forward to making history by tracking the evolution of this comet as it swings around the Sun.”

When comets approach the Sun, volatile gases evaporate from their surface carrying fountains of tiny dust particles with them. In part, these gases and dust remain bound to the nucleus by gravity forming the comet’s coma.

This early onset of cometary activity offers scientists the opportunity to study dust production and structures within the coma at an early stage of the mission.

In addition, tracking the periodic changes in brightness reveals the nucleus is rotating every 12.4 hours – about 20 minutes shorter than previously thought.

“These early observations are helping us to develop models of the comet that will be essential to help us navigate around it once we get closer,” says Sylvain Lodiot, ESA Rosetta spacecraft operations manager.

OSIRIS and the spacecraft’s dedicated navigation cameras have been regularly acquiring images to help determine Rosetta’s exact trajectory relative to the comet.

Using this information, the spacecraft has already started a series of manoeuvres that will slowly bring it in line with the comet before making its rendezvous in the first week of August.

Detailed scientific observations will then help to find the best location on the comet for the Philae lander’s descent to the surface in November.

Rosetta will be the first mission in history to rendezvous with a comet, escort it as it orbits the Sun, and deploy a lander to its surface.

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